1. Field of the Invention
The present invention is related to an improvement in a III-V-group compound semiconductor device and more particularly to an improvement in a III-V-group compound semiconductor device including a channel layer and a barrier layer that form a hetero-interface.
2. Description of the Background Art
In the case that III-group nitride semiconductors as typified by GaN, AlGaN, InGaN, and the like are used for electronic devices, it is expected that the excellent properties of those materials can realize good device properties such as a high withstand voltage, a high speed operation, a high heat resistance, a low on-resistance, and the like. Therefore, electronic devices are now being developed with using III-group nitride semiconductors instead of the Si material. Particularly regarding a field effect transistor (FET), the on-resistance of the FET can be reduced with a two-dimensional electron gas formed in the vicinity of a hetero junction interface of AlGaN/GaN. Like this, there have been proposed various device structures that utilize hetero junction interfaces.
FIG. 3 is a schematic cross-sectional view of a conventional hetero junction type field effect transistor (HFET) that utilizes III-group nitride semiconductors (refer to Technical Report of IEICE, ED2001-185, pages 7-12). This HFET is a planar-type device and includes a buffer layer (or crystal-nucleation layer) 303, a GaN channel layer 304 and a AlGaN barrier layer 305 stacked in this order on a substrate 302 of sapphire, Si, SiC, or the like. Formed on AlGaN barrier layer 305 are a source electrode 306, a gate electrode 307 and a drain electrode 308. Between source electrode 306 and drain electrode 308, the upper surface of barrier layer 305 and the upper surface of gate electrode 307 are covered with a protective layer formed of an insulator film of SiN or the like.
When the above-described III-group nitride semiconductor HFET is operated for a long-term period, there arise a problem that the on-resistance of the HFET device is increased gradually as compared to the initial state of the device and then the source-drain current is decreased gradually.
The so-called current collapse phenomenon is well known regarding the III-group nitride semiconductor HFET, in which the sheet carrier density in the channel region is decreased when the source-drain voltage is applied, as compared to the resting state of the device. In order to suppress this phenomenon, it has been attempted to make protective layer 309 (see FIG. 3) for example have a function for suppressing the current collapse. Even if such a countermeasure is provided, however, it is difficult to avoid the problem that the drain current is decreased as mentioned above.
In order to investigate the cause of this problem, the present inventor has conducted the SIMS (secondary ion mass spectroscopy) on an HFET device that has been deteriorated in its properties. As a result, it has been found that the concentration of hydrogen atoms becomes higher in the semiconductor layers, particularly in a region from the upper surface side of the AlGaN barrier layer to the vicinity of the hetero junction interface and GaN channel layer, in the deteriorated HFET device as compared with in a non-deteriorated device.
It has also been found by the SIMS that hydrogen atoms are contained at a high level concentration in the order of 1020/cm3 inside the insulator film covering the upper surface of the device. While the insulator film analyzed at this time was deposited by radio-frequency sputtering, there is a sufficient possibility that hydrogen atoms are contained in the order of 1019-1021/cm3 depending on the deposition condition in the insulator film deposited even by using one of the other deposition methods such as an electron beam evaporation method and a CVD (chemical vapor deposition) method that are often used in processes for forming electronic devices. This hydrogen concentration level in the order of 1020/cm3 is higher with about two digits as compared to the concentration (in the order of 1018/cm3) of hydrogen atoms contained in the III-nitride crystal layers such as the AlGaN barrier layer and GaN channel layer.
Therefore, it is presumed that hydrogen atoms migrate or diffuse from the insulator film containing hydrogen atoms at a high concentration to the interior of the HFET device under the condition that a strong electric field is generated toward the substrate side of the device when the device is in its operation state. At this time, defects due to dissociation of hydrogen bonds in the insulator film are increased and then electric charges are localized at those defects. As a result, on a principle similar to the well-known current collapse phenomenon, the electron concentration is relatively reduced in the two-dimensional electron gas (hereinafter referred to as “2DEG”) generated in the vicinity of the hetero-interface of the AlGaN barrier layer/GaN channel layer and then the resistance component is increased in the current channel of the HFET. It is thus presumed that this is the model in which the current decreases between the source and drain.